The present invention relates to a method and an apparatus for fabricating an electronic circuit device and particularly, to a method of soldering at least two different materials or members of such circuit device without using flux. The present invention further relates to a solder ball to be used therein and to a alignment mark for aligning the members to be connected.
When two different materials or members are to be soldered together, it is usual to use flux or cream containing flux to remove oxide films formed on surfaces of such members, maintain these surfaces clean by preventing oxidation thereof and promote wettability of solder on these surfaces. However, when such flux or flux-containing cream is used, there is a tendency of void generation due to evaporation of flux in soldering operation, as illustrated in FIG. 1, which causes strength and hence reliability of soldered junction to be lowered.
Further, in such conventional technique, there is an environmental pollution problem, particularly, such as destruction of ozone layer, due to removal of flux since flux must be washed out by using organic solvent such as chloric solvent or freon after the soldering is completed. Further, it is very difficult to completely wash out flux although not impossible.
In order to solve such problem, a low residual, low activity flux requiring no washing operation has been proposed. Since, however, such low activity flux is easily oxized when heated in atmosphere, a soldering operation has to be done in a belt furnace filled with N.sub.2. Such furnace is partitioned by four shutters into an inlet gas purge chamber, a heating, melting and bonding chamber in which air is substituted by N.sub.2 to maintain oxygen concentration in the second chamber as low as about 70 ppm and an outlet gas purge chamber.
Since, although this bonding method is easy to use, it has inherent problems mentioned above, fluxless soldering using ion beam has been proposed in J. Vacuum Science Technology, 20(3), March 1982, pp 359-363. In a case of soldering without flux, in order to prevent oxide film from growing on a solder surface after being cleaned by sputtering, an alignment of members to be soldered and soldering material and a heating of soldering material must be performed in non-oxidizing atmosphere which is not practical in views of facility therefor which is large and complicated and technical difficulties of alignment of members itself in such facility.
For example, Japanese Kokai(P) 58-3238 discloses a fluxless soldering method in which two members to be bonded are juxtaposed in a vacuum chamber with solder surfaces thereof being upside, cleaned by ion irradiation, aligned by overlapping them and then irradiated with ion beam to melt it.
Since, in this method, the cleaning, aligning and heating operations are all performed within the vacuum chamber, workability and producibility are very low. Particularly, in the aligning step, an alignment device must be provided within the vacuum chamber, for picking at least one of the members up, turning it over, carrying it onto the other member and aligning a number of bonding portions thereof. This means that the size of the vacuum chamber becomes large causing cost of the whole apparatus to be increased, efficiency to be low and possibility of contamination of the vacuum chamber to be high. Further, due to the use of ion beam, there is a limit in thermal capacity thereof causing simultaneous heating of large substrates to be difficult.
As another example, Japanese Kokai(P) 3-171643 discloses a method in which an atom or ion beam irradiation device and a post processing device for aligning bonding portions and heating solder are separately provided. An interior of the post processing device is filled with inert gas. Two members to be bonded are aligned in a prenum chamber of the post processing device and heated to temperature lower than melting point of solder under pressure to temporarily fix them and transported to a heating/melting chamber of the post processing device to perform soldering.
Although this method is advantageous in that the aligning device is not required in the ion beam irradiation device, the post processing device must be kept of vacuum pressure and the alignment and soldering operations must be done therein.
In recent computer, etc., a number of fine connections using fine solder balls for the so-called flip-chip connection are required. It has been very difficult to bond a number of parts each including such number of connections within an inert atmosphere in such vacuum chamber with high alignment. Further, transportation of such parts with respect to the vacuum chamber and evacuation thereof, etc., are troublesome, causing workability to be low.